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Triple Constraints Model

The triple constraints model has been one of the main staples for teaching project management for as long as I can remember. The model is generally represented by a triangle with Scope on the horizontal leg, Time on the left leg, Cost or Resources on the right leg and Quality in the center of the triangle.

There are variations of the triple constraints theme and name, but the concept is generally the same. The idea has to do with priorities in a project. The stakeholder must decide, in regards to a particular deliverable, which of the three constraints of scope, time and cost is most flexible, medium flexible, and least flexible. Let’s imagine a stakeholder who decides that, for a particular deliverable, time is the most flexible, scope is medium flexible and cost is least flexible. This would mean that, if need be, they would prefer a deliverable be later than originally planned as opposed to the deliverable costing more or having fewer features.

The rule is: if one of the triple constraints is changed, it will most likely affect one or both of the other constraints in addition to affecting the quality of the deliverable. An effective way to demonstrate this concept is to say to the stakeholder, “I have three cards: better, faster, and cheaper. You can only have two cards. Which of the two cards do you want?”

Upon reviewing A Guide to the Project Management Body of Knowledge (PMBOK® Guide)– 4th Edition, which came out in December, 2008, and will be the basis of the Project Management Professional (PMP®) certification test beginning June 30th, 2009, I did a search for the term “triple constraints model” and it was nowhere to be found. In fact, I couldn’t find the word “triple” in the PMBOK® Guide. The closest reference I could find in the PMBOK® Guide– 4th Edition to the triple constraints concept was under Risk – section 11.3, page 289. Although the triple constraints model may not be in the PMBOK® Guide,I believe it to be a Best Practice and a great tool for eliciting requirements and managing change control.

The PMBOK® Guidedoes have many references to constraints, and rightfully so. Constraints are a major consideration in project planning and execution. Theory of Constraints by Dr. Eliyahu M. Goldratt is an excellent book for exploring the impact of constraints. I just find it odd that the PMI® has left out this valuable model for teaching. What are your thoughts?

14 comments

As I recall the ‘official’ answer for the PMP Exam, Quality was considered ‘free’. That is, increased quality, at least when done the right way, paid for itself, neither costing nor saving money. Maybe that is why the triple constraint is not in the PMBOK. Of course, I would argue that an organizational obsession with quality differs with a project quality implementation. While an organization that commits to quality may not incur much cost or even save money, everybody has been on a project where just getting it done is all that counts. ‘We cannot afford to do it right, but we we will have to afford doing it again’ is a common description of that phenomenon.

Darrell,
There is a reference in the first paragrap on page 289 about …”project constraints of cost, schedule, scope, and quality” without refering to the word “triple.” Could it be that there are now 4 constraints (quality added) which changes them to quadruple constraints?
Jerry

Jerry is correct. In the 4th edition of the PMBOK PMI changed the triple constraint. They added the completing demands of risk, quality, and resources to the original ones of schedule, scope, and cost. See chapter 3, PM Processes.

As we teach project management principles to today’s new project managers, we need to take off the blinders and realize that tomorrow’s modern project manager needs to come to grip with more than just the traditional “triple constraints”, plus Quality.

Realistically, we will always be pounded on by stakeholders for schedule (time), budget (cost) and scope. And now, PMI has finally placed Quality on the same playing level as time, cost and scope. For what is the point of bringing a project in on time, on budget, and within scope if what you deliver is a piece of crap.

But, now we propose that 2 more priorities be given “equal” weight. One is Risk. We typically draw a circle around the triple constraints and label the circle “Risk”, as risk pushes on all three sides of the triple constraints. The second proposed priority is customer satisfaction. After all, happy customers are repeat customer, and repeat customers put dollars in the bank. And when the day is over, it is really all about the money.

We are now using a 6-sided figure to represent the priorities that face us in our projects. So, on time, within budget, within scope, good quality, manage the risks that can crush your project, and achieve customer satisfaction; these are the revised priorities that face today’s modern project manager.

The triple constraints I learned were a triangle made up of time, cost and quality making up the three sides or constraints. Changes to any one side would have an impact to at least one of the other two sides. While scope is a constraint within a project, changes to scope will almost always have an impact to all three of the other constraints. In other words, the size and shape of the project contained by the three constraints of time, cost, and quality are determined by the scope of the project. This concept is correctly located in the Risk section of the PMBOK. The question is: does the PMBOK adequately communicate the interdependence between the four constraints?

I am enjoying all the inputs and there all very good stuffs. The originating thought on the TBQ concept by Darrell has brought out all these sound inputs which are very educative for very new or aspiring Project Management students like me.

Though most of the discussion above is related to PMI/PMBOK, my input here is from PRINCE2 which suggests 6 project management constraints:

Time, Cost, Quality, Scope, Risks and Benefits

If you see this same thing from Agile project management perspective, I would say that scope is pretty much a non-existant as a constraint, as scope is something that is assumed to change (due to business value/priorities)

Hi, from the PMP exam point of view, I think PMBOK v4 will hence forth be the reference and our answers should dwell around the 6 ‘competing demands’ rather than the ‘Triple Constraints’.
Am I right?
Some one from the PMI may kindly answer?

In agreement with comment from “pd”.
Scope, includes the definition of quality, then, the schedule, and definition of the budget.
Risk management is a tool to be used in all aspects of the project, i.e., in all triple constraint points.Problem is really, to start with a very good definition of the scope which incluedes important issues like customer satisfaction. Schedule will depend on the resources assigned, and therefore will depend on the budget.
The PMB should discuss the triple constraint as one can affect the other two arms of the triangle. The worse scenario is a poor definition of the scope with no risk management.
Does anybody know what is the triple constraint status in the new edition of the PM/PMBOK of December 2012?

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